The Dirty Secrets Saved in Dead Birds’ Feathers

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A comparison of horned larks from the early 20th century in the collection at the Field Museum in Chicago. The specimens on the left were collected inside the manufacturing belt in Illinois. The specimens on the right were collected along the West Coast, away from industrial areas.CreditCarl Fuldner and Shane DuBay

The specimens that were put away around the start of the 20th century are far grimier than the ones from more recent decades. And now, climate scientists and historians can thank museum curators for not having tidied them up before storing them.

That’s because the soot preserved on their feathers contains missing pixels in a picture of urban air pollution over 135 years, according to a study published Monday in Proceedings of the National Academy of Sciences. Their feathers reveal historical air quality in Pennsylvania, Ohio, Indiana, Michigan, Illinois and Wisconsin, dating back more than 70 years before federal monitoring standards were established.

When two graduate students at the University of Chicago measured black carbon clinging to the chests and bellies of more than 1,300 birds in collections at museums in America’s Rust Belt, they found that the dirt on their plumage contained a record of America’s coal use over time.

Between 1880 and 2015, the filth on their feathers undulated with social changes and environmental policy. The dirtiest birds flew through the skies just before 1910, at the height of industrialization. But during the Great Depression and after the middle of the 20th century, birds soared with brighter feathers as people burned less coal and home heating transitioned to natural gas.

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A collection of red-headed woodpeckers at the Field Museum.CreditCarl Fuldner and Shane DuBay

“We can estimate how much smoke was actually in the atmosphere,” said Shane DuBay, a graduate student in evolutionary biology at The Field Museum and the University of Chicago and co-author of the study. “It might have been worse than the best estimates have predicted.”

As early as the 1930s, scientists noted the darker feathers of birds collected in the early 20th century. Some wondered if they were an adaptation to pollution — like the peppered moths of Britain’s Industrial Age. But Mr. DuBay and Carl Fuldner, a graduate student in art history at the University of Chicago and a co-author, used a special microscope to look closely at the feathers. They were just dirty — dusted in the same tiny particles of black carbon that were there a hundred years ago.

The team realized this dirty mess was a beautiful opportunity. Birds molt every year, so grimy feathers contained no more than a year’s worth of black carbon. With enough samples, they could place these birds back in their historical context.

They quantified the sootiness by taking pictures of the chests of sparrows, woodpeckers, larks and towhees and measuring how well they reflected light. Black carbon isn’t very reflective — which is why it’s been a problem in the environment. It absorbs sunlight and generates heat, leading to alterations in cloud cover and precipitation or acceleration of the melting of snow and ice.

The team found that dirty feathers corresponded to historical reports and other atmospheric measurements — and they were more localized in time and space. They hope that scientists and policy makers can use the data to inform climate models and environmental policy. And the birds, once dead and void of context in museum drawers, now play a part in environmental history.

Mr. DuBay and Mr. Fuldner have more questions: Did soot affect the health of birds and other wildlife, as it has in humans? Did the dirt alter their reproductive strategies? And could birds in other industrialized nations of the past and present like England or China, also reveal hidden measures of air pollution?

“These collections are hidden treasures, and who knows what other questions we’ll be able to ask by getting into these museums and exploring,” Mr. DuBay said.

Correction:

An earlier version of this article described natural gas incorrectly. It is in fact a fossil fuel, not an alternative source of energy. Additionally, it referred incorrectly to the journal in which the study was published. It was the Proceedings of the National Academy of Sciences, not the Proceedings of the Natural Academy of Sciences.

A version of this article appears in print on , on Page D2 of the New York edition with the headline: Flight Recorders: Dead Birds Tell Tales From a Polluted Past. Order Reprints | Today’s Paper | Subscribe